1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission and, more particularly, to hydraulic control of a so-called clutch-to-clutch (engagement changeover) shift that changes speeds by engaging one frictional engagement element and releasing another frictional engagement element, especially, a clutch-to-clutch downshift.
2. Description of the Related Art
In normal running vehicle operation changes between a power-on condition where the accelerator pedal is depressed so that drive force is transmitted from the engine to the wheels, and a power-off condition where the accelerator pedal is released so that no drive force is transmitted from the engine to the wheels. Operation will also change between a high-vehicle-speed and high-torque running condition and a low-vehicle-speed and low-torque running condition. The automatic transmission of the vehicle is controlled to shift in accordance with these various running conditions. The shifts required of the automatic transmission include, for example, a power-on downshift, that is, a downshift in response to depression of the accelerator pedal as in kick-down, and a power-off upshift, that is, an upshift in response to release of the accelerator pedal.
Japanese examined patent application publication No. HEI-6-33817 discloses a hydraulic control method for the aforementioned clutch-to-clutch (engagement changeover) shift. This method determines that a frictional engagement element engaged in the shift has reached post-shift input-output rotational synchronism upon detection that its input-output rotational speed ratio has reached approximately 1.0. For a power-on downshift or a power-on upshift, the hydraulic control method maintains the operating pressure supply to the post-shift speed frictional engagement element at a pressure slightly lower than the engagement starting pressure, during a period from the start of the shift until input-output rotational synchronism is determined. Upon detecting the input-output rotational synchronism, the operating pressure is raised to a predetermined engaging pressure, thereby precisely engaging the post-shift clutch without a time lag.
The above-described conventional hydraulic control method effects a power-on downshift by releasing the pre-shift frictional engagement element to allow the engine speed to rise, and maintaining the engaging pressure at a low level until the input and output rotational speeds of the post-shift frictional engagement element become synchronous, and engaging the post-shift frictional engagement element after the input-output synchronization. However, although this control is effective at high vehicle speeds, because the engine speed rises rapidly (the rotational speed change is large), the control may fail to timely complete engagement of the post-shift frictional engagement element at low vehicle speed or low torque because the rotational speed change is small, giving an unpleasant sensation to the driver.
Theoretically, it is also possible to simultaneously control the engaging fluid pressure and the releasing fluid pressure independently of each other, while the vehicle is at a low speed. However, this method would also require separate control logics and, as a consequence, an increased memory capacity. Moreover, the aforementioned conventional hydraulic control requires a separate control logic for a power-off downshift instructed by, for example, a manual downshift operation, and therefore also requires additional memory capacity.